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AR# 20565

10.1 Timing - Virtex-4 - Why is the IDELAY DEFAULT delay value so large?


When retargeting designs from Virtex-II to Virtex-4 by setting the IDELAY component in the IOB to DEFAULT to be backward-compatible to the Virtex-II IOB delay, the Tidockd and time through IOBUF both increased significantly, causing timing problems.


The Tidockd (IDELAY Default) values increased to create a zero or negative hold time for the input pads.

To work around this issue, use the Fixed Delay Mode of the IDELAY component to reduce the value.

For more information on how to use the IDELAY component for Fixed Delay Mode, see the Virtex-4 User Guide at:


AR# 20565
Date 12/15/2012
Status Active
Type General Article
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